The UN has launched a counter-offensive against moves to liberalise drug laws around the world, warning that cannabis legalisation poses a grave danger to public health.

The UN body for enforcing international drug treaties, the International Narcotics Control Board (INCB), voiced concern over “misguided initiatives” on cannabis legalisation in Uruguay and the US states of Colorado and Washington that fail to comply with international drug conventions.

“Drug-traffickers will choose the path of least resistance, so it is essential that global efforts to tackle the drug problem are unified,” said Raymond Yans, INCB president.

“When governments consider their future policies on this, the primary consideration should be the long-term health and welfare of the population.”

He said the UN was concerned about some initiatives aimed at the legalisation of the non-medical and non-scientific use of cannabis that posed “a very grave danger to public health and wellbeing” – the very things international drug conventions had been designed to protect.

But the INCB report argues against such “alternative drug regimes”, claiming legalisation would not collapse “underground markets”, but instead would lead to much greater use of such drugs and higher levels of addiction.

Pointing to the history of alcohol and tobacco markets, the report says that despite legalisation there is still a thriving black market for cigarettes in many countries. It says up to 20% of Britain’s domestic cigarette market consists of smuggled cigarettes, while they represent 33% of all domestic cigarette consumption in Canada.

Alcohol, despite being legal, is also responsible for far more arrests than illegal drugs. In the US there were 2m alcohol-related arrests in 2012 compared with 1.6m related to illegal drugs.

“One reason for those higher alcohol-related costs is that in many countries alcohol abuse is far more prevalent than the abuse of substances under international control,” the report says.

The UN remains most concerned about the scale of illicit opium poppy cultivation in Afghanistan, which set records in 2013 reaching 209,000 hectares, a 36% increase compared with 154,000 hectares in 2012.

“The country remains the centre of the illicit manufacture of heroin and its importance as a source of cannabis resin for the world markets is growing. The situation seriously endangers the aims of the international drug control treaties,” the INCB report says.

It repeats its warnings on legal highs or new psychoactive substances as they are officially known, and says unprecedented numbers and varieties of these synthetic chemical substances are being sold in the developing world as well as Europe.

The UN drugs report also highlights the significance of widespread prescription drug abuse in the US and says that “takeback” days promoting their safe disposal are not enough to tackle this growing trend.